In chemical industries, the controlling of the flow of highly corrosive chemicals, such as various acids including hydrofluoric acid, hydrochloric acid, and similar materials, as well as highly corrosive bases such as hydrogen peroxide and similar materials, continues to be a vexing problem because leakage of such liquids out of the flow systems at the valves continues to be a substantial problem. Various plastic poppet valves have been used and weir valves have been used, using diaphragms and other all plastic parts, but prevention of leakage has not been completely accomplished. Previously known clamp valves, closing a flexible tube, have not proven reliable in completely closing the flow passage.